Impedance control devices, also called tuners, are devices of which the impedance, presented to the outside world, can be changed. This is done by either manually changing one or more device properties or by changing one or more properties via an electronic means. Such property is also referred to as a parameter of the tuner. The device typically has one port or two ports, but could in principle have more ports. Via the port these devices are connected to the outside world and provide a controllable impedance to the outside world. In most cases a port is a physical connector through which the impedance control device can be connected to another device. However, the port does not need to be limited to a connector. The port defines a boundary between the impedance control device and the outside world. Amongst others, a port can be a pad of an integrated circuit (IC). The impedance range the impedance control device can provide depends on the physical properties of the device.
Impedance control devices are well established in source- and load-pull measurement set-ups or measurement systems. These set-ups are used to determine the impedances to be presented at the input and/or output of a device under test in order to optimize one or more of its performance characteristics, e.g. the delivered output power, power added efficiency and other. In this case the device under test is typically a transistor or an amplifier under test for which the optimal input and output match is determined during amplifier design. These set-ups are also used to characterize the behaviour of devices, e.g. transistors, diodes, amplifiers, mixers etc. under realistic test conditions or to verify and/or improve their model, used in computer aided engineering (CAE) tools.
Impedance tuners can roughly be divided in two different classes, namely active tuning systems and passive tuning systems.
Passive tuning devices are typically based on a mechanical structure, like a probe or resonator that is moved, or by changing stub lengths by shorting diodes, or by changing capacitor values, typically all on one or another transmission line. The position of the probe, consisting of a depth and longitudinal length, is a set of parameters of the impedance tuner that will determine the impedance.
In active tuning the tuner, considered in broad sense, internally generates in one or another way power to influence the signal coming out of the DUT and as such to modify the impedance seen by the DUT. Active impedance tuning systems, in contrast to passive tuning systems, allow generating reflection factors at the device under test (DUT) close to one and even larger, taking advantage of the internal generated power that can compensate for losses.
Two classes of active load pull systems exist: either with open loop or with closed loop.
Active tuners with open loop use a HF source and inject power towards the DUT output in a phase coherent way with the HF source providing the input signal to the device under test. This can be realized in different ways, e.g. by splitting the input source, followed by amplifying or attenuating and phase shifting it, or by using a second source which is controllable in amplitude and phase and phase locked to the source at the input. Power is generated at the DUT output to counteract at a selected frequency the power coming out of the DUT. By setting the level and the phase of the power, generated by that source, an impedance is actively created at the selected frequency. The main drawback is that when the power coming out of the DUT changes, one needs to adapt the power and phase of the source. For this type of tuner it is not possible to set a parameter that directly translates into an impedance. One needs the interaction of the wave coming out of the device under test with the active impedance tuner.
In an active tuner with a closed loop the signal coming out of the DUT is coupled off, filtered at a selected frequency, amplified or attenuated, modified in phase and re-injected towards the DUT. Depending on the filtering bandwidth the impedance value presented by this loop will track variations of the signal coming out of the DUT. For this type of tuner it is possible to determine the loop gain based on the coupler characteristics and the setting of the gain and phase shifts. The latter can be considered as the impedance tuner parameters which determine the impedance provided to the outside world.
Passive tuners, based on moving probes or resonators are typically slow. Active tuners have typically been faster than passive tuners.
Nowadays new tuning solutions are becoming available, which can change the impedance very fast even for passive tuners, e.g. passive tuning based on MEM technology, solid-state technology or active tuning techniques taking advantage of FPGA technology.
Still even using these new tuning solutions the characterization of components is a tedious time-consuming task because it is not possible to exploit the tuner speed. Typically one steps the input power, steps the source and load impedance and performs these measurements at different bias points and frequency points. This is done using one or another computing device, like a computer, stepping the stimuli, acquiring and collecting all the measurements. As such the speed of applying different stimuli (in general sense, also including applying different impedances) and the speed of measurement and the synchronization between them are primordial to characterize devices very fast. As said this is still very time-consuming as soon as an impedance tuner is present.
Eliminating the tuner from the setup, it is possible to characterize components very fast, taking advantage of the capabilities of arbitrary waveform generators, vector signal generators generating broadband modulation signals in combination with synchronous broadband data acquisition channels and vector signal analyzers.
As soon as tuners are involved, presently one needs to step the tuner through its impedances and for each impedance one needs to perform different sweeps.
Hence, there is a need to extend the impedance tuner such that it possibly can run at its maximal allowable speed changing its impedance values while it can be combined with the regular signal generation means and data acquisition means. In other words, a solution is desirable to extend an impedance tuner so that it will act similar to an arbitrary waveform generator generating impedances as function of time instead of a voltage as function of time.